Watch crystal



FIG. 1 is a top plan view of a first embodiment of the design for watch crystal;

FIG. 2 is a front view thereof;

FIG. 3 is a right side view thereof, the left side view being identical;

FIG. 4 is a rear view thereof;

FIG. 5 is a top plan view of a second embodiment of the present invention;

FIG. 6 is a front view thereof;

FIG. 7 is a right side view thereof, the left side view being identical; and,

FIG. 8 is a rear view thereof. 

The ornamental design for a watch crystal, as shown and described. 